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GitHub User: jcarley
Not sure if this is possible. The CORS configuration on S3 would have to allow origins from every single user that is using your API. The examples in this screencast upload files directly from a user's browser straight to S3. The file never goes through the rails application, only metadata about the file does.
You could get fancy and create some sort of registration process, where by the user of the API during the registration process enters the domain (origin) they plan to upload from. Or maybe they have a admin screen where they can change it out. AWS does expose an API where you can change settings on your S3 service. So when your users are in the admin screen, and they change the domain, then your rails app uses the S3 API to change the origin (or add) that the user entered. I hope I explained that well. I'm just spitballing here, never done this myself.